The present invention relates to a chemical amplifying type positive resist composition used in the minute processing of a semiconductor and a novel compound usable as an acid generator in said resist composition.
In general, a lithography process using a resist composition has been adopted in the minute processing of a semiconductor. In lithography, the resolution can be improved with a decrease in wavelength of exposure light in principle as expressed by the equation of Rayleigh""s diffraction limited. A g-line with a wavelength of 436 nm, an i-line with a wavelength of 365 nm, and a KrF excimer laser with a wavelength of 248 nm have been adopted as exposure light sources for lithography used in the manufacture of a semiconductor. Thus, the wavelength has become shorter year by year. An ArF excimer laser having a wavelength of 193 nm is considered to be promising as a next-generation exposure light source, and some of resists for ArF excimer laser are being made practical.
A lens used in an ArF excimer laser exposure machine or an exposure machine using a light-source of shorter wave-length has a shorter lifetime as compared with lenses for conventional exposure light sources. Accordingly, the shorter time required for exposure to ArF excimer laser light is desirable. For this reason, it is necessary to enhance the sensitivity of a resist. Consequently, there has been used a so-called chemical amplifying type resist, which utilizes the catalytic action of an acid generated due to exposure, and contains a resin having a group cleavable by the action of acid.
It is known that, desirably, resins used in a resist for ArF excimer laser exposure have no aromatic ring in order to ensure the transmittance of the resist, but have an alicyclic ring in place of an aromatic ring in order to impart a dry etching resistance thereto. Various kinds of resins such as those described in Journal of Photopolymer Science and Technology, Vol. 9, No. 3, pages 387-398 (1996) by D. C. Hofer, are heretofore known as such resins.
S. Takechi et al., Journal of Photopolymer Science and Technology, Vol. 9, No. 3, pages 475-487 (1996), and JP-A-9-73173 reported that, when a polymer or copolymer of 2-methyl-2-adamantyl methacrylate was used as the resin in a chemical amplifying type resist, 2-methyl-2-adamantyl was cleaved by the action of an acid to act as an positive type and a high dry etching resistance, a high resolution and a good adhesion to a substrate could be obtained. In addition, JP-A-10-274852 reported that the adhesion to a substrate was improved by using a resin having a butyrolactone residue in a part of polymerization units as the resin constituting a chemical amplification type positive resist composition. Further, JP-A-10-319595 described a resist composition containing a resin having a xcex3-butyrolactone-3-yl residue as a protective group for carboxyl group.
On the other hand, since the chemical amplification type resists utilizes the action of an acid, a problem arises that profiles are liable to be bottom-tailed by deactivation of the acid when the substrate is of a basic nature. It is known that this problem can be resolved by adding a much amount of a basic quencher substance. Addition of a much amount of such quencher substance, however, results in decrease of the sensitivity of the resist. In addition, in ArF-exposure, a resist is often applied on a substrate having a low reflection such as an organic or inorganic anti-reflective layer. When such a substrate having a low reflection is used, the profile of the resist generally deteriorated in a taper shape due to light absorption, although dimension uniformity is effectively improved.
One possible mean for lowering the light absorption is to reduce the amount of the acid generator. In this case, however, the sensitivity is generally decreased. Another mean for lowering the light absorption is to use an aliphatic sulfonium salt having a high transparency such as those described in JP-A-7-25846, JP-A-7-28237, JP-A-7-92675and JP-A-8-27102. In the cases of these known aliphatic sulfonium salts, however, a sufficient resolution cannot be obtained and a problem that the profile on a basic substrate becomes bottom-tailed shape cannot be dissolved. Therefore, the chemical amplification type resists containing a conventional acid generator had a problem that performances, particularly the profile, are varied depending on the kind of the substrate.
An object of the present invention is to provide a chemical amplification type positive resist composition, which contains a resin component and an acid generator, which is suitable to use in excimer laser lithography with ArF, KrF or the like, particularly in lithography with a light having a wavelength of 220 nm or lower, for example, ArF excimer laser light, and which is superior in sensitivity and resolution confering a good profile.
Another object of the invention is to provide a compound useful as an acid generator in such a chemical amplification type positive resist composition.
The present inventors have found the fact that the transmittance and the resolution can be improved by using a combination of certain kinds of acid generators. Thus, the present invention has been completed.
The present invention provides a chemical amplifying type positive resist composition comprising an aliphatic sulfonium salt represented by the following formula (I): 
wherein either Q1, Q2, Q3 and Q4 independently represent an alkyl group having 1 to 6 carbon atoms or a cycloalkyl group having 3 to 10 carbon atoms, or Q1 and Q2 and/or Q3 and Q4 independently form, together with the adjacent sulfur atom, a heterocyclic group which has 2 to 8 carbon atoms and which may further have an oxygen atom or a sulfur atom, and m represents an integer of 1 to 8;
at least one onium salt selected from the group consisting of a triphenylsulfonium salt represented by the following formula (IIa) and a diphenyliodonium salt represented by the following formula (IIb): 
wherein Q5, Q6, Q7, Q8 and Q9 independently represent a hydrogen atom, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms, and p and q represent integer of 1 to 8; and
a resin which contains a polymerization unit having a group unstable to an acid, and which is insoluble in alkali by itself but becomes soluble in alkali by the action of an acid.
The aliphatic sulfonium salt represented by the above formula (I) is a novel compound never described in literature. Therefore, the invention also provides the sulfonium compound represented by the above formula (I).
The acid generator used in the chemical amplification type resist composition is a substance that decomposes to generate an acid by acting a radiation such as a light or an electronic ray on the substance itself or a resist composition containing the substance. In the resist composition of the invention, both of an aliphatic sulfonium salt represented by the above formula (I), and at least one onium salt selected from the group consisting of a triphenylsulfonium salt represented by the above formula (IIa) and a diphenyliodonium salt represented by the above formula (IIb). Such acid generators are used together.
In the formula (I), either Q1, Q2, Q3 and Q4 independently represent an alkyl group having 1 to 6 carbon atoms or a cycloalkyl group having 3 to 10 carbon atoms. Alternatively, Q1, Q2 and the sulfur atom bonding to them, and/or Q3, Q4 and the sulfur atom bonding to them may form a heterocyclic group which has 2 to 8 carbon atoms and which may further have an oxygen atom or a sulfur atom. When the alkyl group has 3 or more carbon atoms, the group can be straight-chained or branched. Typical examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl and the like. Typical examples of the cycloalkyl group include cyclopentyl, cyclohexyl, cycloheptyl and the like. Typical examples of the heterocyclic groups formed by Q1, Q2 and the sulfur atom bonding to them, and those formed by Q3, Q4 and the sulfur atom bonding to them include ethylene sulfide, trimethylene sulfide, tetrahydrothiophene, tetrahydrothiopyran, thioxane, dithian, tetrahydrothiophene-3-one, tetrahydrothiopyran-4-one and the like. In the formula (I), m representing the number of carbon atoms in an alkane moiety constituting a perfluoroalkanesulfonate anion is an integer of 1 to 8. Typical examples of the moiety corresponding to the perfluoroalkanesulfonate anion include trifluoromethanesulfonate ion, perfluorobutanesulfonate ion, perfluorooctanesulfonate ion and the like.
The aliphatic sulfonium salt represented by the formula (I) has a high transmittance with respect to a light having a wavelength of 220 nm, such as ArF excimer laser light having a wavelength of 193 nm, since the groups constituting the sulfonium cation is non-aromatic groups. Therefore, when such an aliphatic sulfonium salt is used as an acid generator, a resist composition containing the acid generator has a smaller rate of absorption for a short wavelength exposure light as described above, and can avoid a bottom-tailed profile.
The aliphatic sulfonium compound represented by the formula (I) can be produced according to the known method. For example, they can be produced according to the following scheme by applying a method described by J. V. Crivello et al., Journal of Polymer Science, Polymer Chemistry Edition, Vol. 17, 2877-2892 (1979): 
wherein Q1, Q2, Q3, Q4 and m are as defined above, X represents a halogen such as bromine and iodine, and M represents an alkali metal such as sodium and potassium or silver.
An aliphatic sulfonium salt represented by the formula (I) can be obtained by acting a sulfide compound of the formula (A2) on a dihalogenoacetone of the formula (A1) to give a sulfonium halide of the formula (B), followed by further acting a metal salt of a perfluoroalkanesulfonic acid of the formula: CmF2m+1SO3M . These reactions are carried out in an appropriate solvent, such as acetone, acetonitrile, nitromethane or the like. The sulfide compound of the formula (A2) is used in an amount preferably of 1.8 to 3 moles, more preferably of 2.0 to 2.2 moles, based on 1 mole of the dihalogenoacetone corresponding to the formula (A1). The metal salt of a perfluoroalkanesulfonic acid of the formula: CmF2m+1SO3M may be used preferably in an amount of 0.8 to 1.2 mole, more preferably 0.9 to 1.1 mole, based on 1 mole of the sulfide compound of the formula (A2) used for the production of the sulfonium halide of the formula (B). After completion of the reaction, the aliphatic sulfonium salt can be obtained by removing the generated metal halide salt by filtration or the like and subjecting the solution to a post-treatment such as concentration, recrystallization or others.
Specific examples of the aliphatic sulfonium salt represented by the formula (I) include the following compounds:
(2-oxo-1,3-propanediyl)bis(dimethylsulfonium) bis(trifluoromethanesulfonate),
(2-oxo-1,3-propanediyl)bis(dimethylsulfonium) bis(perfluorobutanesulfonate),
(2-oxo-1,3-propanediyl)bis(dimethylsulfonium) bis(perfluorooctanesulfonate),
(2-oxo-1,3-propanediyl)bis(diethylsulfonium) bis(perfluorobutanesulfonate),
(2-oxo-1,3-propanediyl)bis(dibutylsulfonium) bis(trifluoromethanesulfonate),
(2-oxo-1,3-propanediyl)bis(dibutylsulfonium) bis(perfluorobutanesulfonate),
(2-oxo-1,3-propanediyl)bis(dibutylsulfonium) bis(perfluorooctanesulfonate),
(2-oxo-1,3-propanediyl)bis(diisopropylsulfonium) bis(perfluorobutanesulfonate),
(2-oxo-1,3-propanediyl)bis(tert-butylmethylsulfonium) bis(perfluorobutanesulfonate),
(2-oxo-1,3-propanediyl)bis(cyclohexylmethylsulfonium) bis(perfluorobutanesulfonate),
1,1xe2x80x2-(2-oxo-1,3-propanediyl)bis(tetrahydrothiophenium) bis(trifluoromethanesulfonate),
1,1xe2x80x2-(2-oxo-1,3-propanediyl)bis(tetrahydrothiophenium) bis(perfluorobutanesulfonate),
1,1xe2x80x2-(2-oxo-1,3-propanediyl)bis(tetrahydrothiophenium) bis(perfluorooctanesulfonate),
1,1xe2x80x2-(2-oxo-1,3-propanediyl)bis(tetrahydrothiopyranium) bis(perfluorobutanesulfonate),
1,1xe2x80x2-(2-oxo-1,3-propanediyl)bis(1,4-thioxolanium) bis(perfluorobutanesulfonate),
1,1xe2x80x2-(2-oxo-1,3-propanediyl)bis(4-oxotetrahydrothiopyranium) bis(perfluorobutanesulfonate) and the like.
In the present invention, at least one onium salt selected from the group consisting of compounds of the formula (IIa) and the formula (IIb) is used as the acid generator together with the aliphatic sulfonium salt of the formula (I). In these onium salts, Q5, Q6, Q7, Q8 and Q9 respectively represent a hydrogen atom, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms. When the alkyl group or the alkoxy group has 3 or more carbon atoms, such group can be straight-chained or branched. Typical examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl and the like. Typical examples of the alkoxy group include methoxy, ethoxy, propoxy, butoxy and the like. In the formulae (IIa) and (IIb), p and q representing the numbers of carbon atoms in alkane moieties constituting perfluoroalkanesulfonate anion are integers of 1 to 8.
The triphenylsulfonium salt represented by the formula (IIa) and the diphenyliodonium salt represented by the formula (IIb) can be commercial products thereof, if available. Otherwise, they can be produced according to the conventional process. As to the process for producing the triphenylsulfonium salt (IIa), following processes can be exemplified: a process in which the corresponding triphenylsulfoniumbromide is reacted with a silver perfluoroalkanesulfonate, a process in which the corresponding diphenylsulfoxide is reacted with a benzene compound and a perfluoroalkanesulfonic acid in the presence of trifluoroacetic anhydride according to the description in Chemical and Pharmaceutical Bulletin, Vol. 29, 3753 (1981), a process in which a corresponding aryl Grignard reagent is reacted with thionyl chloride, then with a triorganosilyl halide to give a triarylsulfonium halide, followed by a reaction with a silver perfluoroalkanesulfonate according to the description in JP-A-8-311018, and so on. Compounds of the formula (IIa) wherein at least one of Q5, Q6 and Q7 is hydroxyl group can be produced by treating a triphenylsulfonium salt having a tert-butoxy group on the benzene ring with a sulfonic acid having the same anion as that of the triphenylsulfonium salt to eliminate the tert-butyl group according to the description in the same JP-A-8-311018.
As to the process for producing the diphenyliodonium salt (IIb), following processes can be exemplified: a process in which iodyl sulfate is reacted with a corresponding aryl compound, followed by addition of a perfluoroalkanesulfonic acid according to the description in Journal of American Chemical Society, Vol. 81, 342 (1959), a process in which concentrated sulfuric acid is added dropwise to a mixture of a corresponding aryl compound, acetic anhydride and potassium iodate to cause a reaction, followed by addition of a perfluoroalkanesulfonic acid, a process in which a reaction product obtained by adding iodine and trifluoroacetic acid to a mixed solution of acetic anhydride and fuming nitric acid is reacted with a corresponding aryl compound, followed by addition of a perfluoroalkanesulfonic acid, and so on.
Specific examples of the triphenylsulfonium salt corresponding to the formula (IIa) and the diphenyliodonium salt corresponding to the formula (IIb) include the following compounds:
triphenylsulfonium trifluoromethanesulfonate,
triphenylsulfonium perfluorobutanesulfonate,
triphenylsulfonium perfluorooctanesulfonate,
4-methylphenyldiphenylsulfonium trifluoromethanesulfonate,
4-methylphenyldiphenylsulfonium perfluorobutanesulfonate,
4-hydroxyphenyldiphenylsulfonium perfluorobutanesulfonate,
4-methoxyphenyldiphenylsulfonium perfluorobutanesulfonate,
tris(4-methylphenyl)sulfonium perfluorobutanesulfonate,
tris(4-methoxyphenyl)sulfonium perfluorobutanesulfonate,
triphenylsulfonium perfluorooctanesulfonate,
4-methylphenyldiphenylsulfonium perfluorooctanesulfonate,
4-hydroxyphenyldiphenylsulfonium perfluorooctanesulfonate,
4-methoxyphenyldiphenylsulfonium perfluorooctanesulfonate,
tris(4-methylphenyl)sulfonium perfluorooctanesulfonate,
tris(4-methoxyphenyl)sulfonium perfluorooctanesulfonate,
diphenyliodonium perfluorobutanesulfonate,
di(4-methoxyphenyl)iodonium perfluorooctanesulfonate,
di(4-tert-butylphenyl)iodonium perfluorooctanesulfonate, and the like.
The resin component constituting the resist composition of the invention contains a polymerization unit having a group unstable to an acid. The resin for use in a chemical amplifying type positive resist is generally alkali-insoluble or hardly alkali-soluble by itself. However, a part of a group therein is cleaved by the action of an acid, and the resin becomes alkali-soluble after the cleavage. The group unstable to an acid in the invention can be various groups conventionally known in the art. Examples of the group unstable to an acid include various esters of carboxylic acid. Examples of the esters of carboxylic acid include alkyl esters such as methyl ester and tert-butyl ester, acetal type esters such as methoxymethyl ester, ethoxymethyl ester, 1-ethoxyethyl ester, 1-isobutoxyethyl ester, 1-isopropoxyethyl ester, 1-ethoxypropyl ester, 1-(2-methoxyethoxy)ethyl ester, 1-(2-acetoxyethoxy)ethyl ester, 1-[2-(1-adamantyloxy)ethoxy]ethyl ester, 1-[2-(1-adamantanecarbonyloxy)ethoxy]ethyl ester, tetrahydro-2-furyl ester and tetrahydro-2-pyranyl ester, alicyclic esters such as isobornyl ester and 2-alkyl-2-adamantyl ester, and the like. Examples of the monomers leading these polymerization units having a carboxylic ester include (meth)acrylic monomer such as methacrylic ester and acrylic ester, and alicyclic monomers having a carboxylic ester bound thereto such as norbornenecarboxylic ester, tricyclodecenecarboxylic ester and tetracyclodecenecarboxylic ester.
Among the polymerization unit having a group unstable to an acid, a polymerization unit of 2-alkyl-2-adamantyl (meth)acrylate is preferable from a viewpoint of resolution of the resist containing it. This polymerization unit can be formed by opening the double bond of (meth)acrylic acid moiety in the 2-alkyl-2-adamantyl acrylate or 2-alkyl-2-adamantyl methacrylate, and is specifically represented by the following formula (III): 
wherein R1 represents hydrogen or methyl and R2 represents alkyl,
The polymerization unit of 2-alkyl-2-adamantyl (meth)acrylate represented by the formula (III) ensures the transmittance of a resist and contributes to the improvement of dry etching resistance due to the presence of an adamantane ring. Further, the 2-alkyl-2-adamantyl in this unit is cleaved by the action of an acid. Hence, this unit contributes to the enhancement of alkali-solubility after exposure to radiation of a resist film. R2 in the formula (I) is alkyl. This alkyl may have, for example, about 1 to 8 carbon atoms. In general, the alkyl is advantageously straight-chained, but it may be branched when the number of carbons is 3 or more. Specific examples of R2 include methyl, ethyl, propyl, isopropyl, butyl and the like. Among them, those having methyl or ethyl as R2 are preferred for the improvement of adhesion between the resist film and the substrate and for the improvement of resolution.
Specific examples of monomers leading the polymerization unit of 2-alkyl-2-adamantyl (meth)acrylate represented by the formula (III) include 2-methyl-2-adamantyl acrylate, 2-ethyl-2-adamantyl acrylate, 2-methyl-2-adamantyl methacrylate, 2-ethyl-2-adamantyl methacrylate and the like. The 2-alkyl-2-adamantyl (meth)acrylate can generally be produced by the reaction of a 2-alkyl-2-adamantanol or a metal salt thereof with an acrylic halide or methacrylic halide.
The resin defined in the invention can also contain another polymerization unit that is not cleaved or is hardly cleaved by the action of an acid in addition to a polymerization unit having a group unstable to an acid as described above. Examples of another possible polymerization unit include those derived from monomers having a free carboxylic acid group such as acrylic acid or methacrylic acid, those derived from aliphatic unsaturated dicarboxylic anhydride such as maleic anhydride or itaconic anhydride, those derived from 2-norbornene, those derived from (meth)acrylonitrile, those derived from various (meth)acrylic esters such as 2-hydroxyethyl (meth)acrylate, 3-hydroxy-1-adamantyl (meth)acrylate, (meth)acryloyloxy-xcex3-butyrolactone, and the like.
Particularly, the polymerization units of 3-hydroxy-1-adamantyl (meth)acrylate and polymerization units of (meth)acryloyloxy-xcex3-butyrolactone in which the lactone ring is unsubstituted or substituted with an alkyl are preferred from the viewpoint of adhesiveness of the resist film to the substrate. The polymerization unit of 3-hydroxy-1-adamantyl (meth)acrylate cited herein means a unit formed by opening the double bond of the (meth)acrylic acid moiety in the corresponding 3-hydroxy-1-adamantyl(meth)acrylate. The polymerization unit of (meth)acryloyloxy-xcex3-butyrolactone in which the lactone ring is unsubstituted or substituted with an alkyl herein means a unit formed by opening the double bond of the (meth)acrylic acid moiety in xcex1-(meth)acryloyloxy-xcex3-butyrolactone which is unsubstituted or alkyl-substituted in the lactone ring, or a unit formed by opening the double bond of the (meth)acrylic acid moiety in xcex2-(meth)acryloyloxy-xcex3-butyrolactone which is unsubstituted or alkyl-substituted in the lactone ring.
The polymerization units derived from 3-hydroxy-1-adamantyl(meth)acrylate, xcex1-(meth)acryloyloxy-xcex3-butyrolactone which is unsubstituted or alkyl-substituted in the lactone ring, and xcex2-(meth)acryloyloxy-xcex3-butyrolactone which is unsubstituted or alkyl-substituted in the lactone ring can be represented, respectively, by the following formulae (IV), (V) and (VI): 
wherein R3 and R4 independently represent hydrogen or methyl, R5, R6 and R7 independently represent hydrogen or alkyl and R8 represents hydrogen or hydroxyl.
The 3-hydroxy-1-adamantyl (meth)acrylate for leading a unit of the formula (IV) is commercially available and can be produced, for example, by reacting the corresponding hydroxyadamantane with (meth)acrylic acid or a halide thereof. The xcex1- or xcex2-(meth)acryloyloxy-xcex3-butyrolactone for leading a unit of the formula (V) or (VI) can be produced by reacting acrylic acid or methacrylic acid with xcex1- or xcex2-bromo-xcex3-butyrolactone in which the lactone ring is unsubstituted or substituted with an alkyl, or by reacting an acrylic halide or methacrylic halide with xcex1- or xcex2-hydroxy-xcex3-butyrolactone in which the lactone ring is unsubstituted or substituted with an alkyl.
All the polymerization unit of 3-hydroxy-1-adamantyl (meth)acrylate represented by the formula (IV), the polymerization unit of xcex1-(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (V) and the polymerization unit of xcex2(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (VI) have a high polarity and confer an improved adhesiveness between the resist film containing any of them to the substrate. In addition, these polymerization units also contribute to the improvement of the resolution of the resist. Furthermore, the polymerization unit of 3-hydroxy-1-adamantyl (meth)acrylate contributes to the improvement of the dry etching resistance of a resist. Moreover, the polymerization unit of xcex2-methacryloyloxy-xcex3-butyrolactone contributes to the improvement of transmittance of the resist.
Examples of the monomers for leading the polymerization unit of 3-hydroxy-1-adamantyl (meth)acrylate represented by the formula (IV) include 3-hydroxy-1-adamantyl acrylate, 3-hydroxy-1-adamantyl methacrylate, 3,5-dihydroxy-1-adamantyl acrylate, 3,5-dihydroxy-1-adamantyl methacrylate and so on. In the formula (V) and the formula (VI), R5, R6 and R7 are respectively hydrogen or alkyl. This alkyl may have about 1 to 6 carbon atoms and when the alkyl group has 3 or more carbon atoms, the group can be straight-chained or branched. Typical examples of the alkyl represented by R5, R6 and R7 include methyl, ethyl, propyl, butyl and the like. Examples of monomers for leading the polymerization unit of xcex1-(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (V) include
xcex1-acryloyloxy-xcex3-butyrolactone,
xcex1-methacryloyloxy-xcex3-butyrolactone,
xcex1-acryloyloxy-xcex2,xcex2-dimethyl-xcex3-butyrolactone,
xcex1-methacryloyloxy-xcex2,xcex2-dimethyl-xcex3-butyrolactone,
xcex1-acryloyloxy-xcex1-methyl-xcex3-butyrolactone,
xcex1-methacryloyloxy-xcex1-methyl-xcex3-butyrolactone and the like.
Examples of monomers leading the polymerization unit of
xcex2-(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (VI) include xcex2-acryloyloxy-xcex3-butyrolactone,
xcex2-methacryloyloxy-xcex3-butyrolactone,
xcex2-methacryloyloxy-xcex1-methyl-xcex3-butyrolactone and the like.
Resins containing a polymerization unit of 2-norbornene have a strong structure, because they have an alicyclic ring directly in the main chain. As the result, they are excellent in dry etching resistance. The polymerization unit of 2-norbornene can be introduced, for example, by a radical polymerization using an aliphatic unsaturated dicarboxylic anhydride such as maleic anhydride or itaconic anhydride together with the corresponding 2-norbornene. Therefore, the polymerization unit of 2-norbornene is a unit formed by opening the double bond therein and can be represented by the formula (VII). The polymerization units of maleic anhydride and the polymerization unit of itaconic anhydride as the polymerization units of the aliphatic unsaturated dicarboxylic anhydrides are units formed by opening the double bonds therein and can be represented by the formulae (VIII) and (IX). 
In the formula (VII), either R9 and R10 independently represent hydrogen, alkyl having 1 to 3 carbon atoms, hydroxyalkyl having 1 to 3 carbon atoms, carboxyl, cyano or the group: xe2x80x94COOZ wherein Z is an alcohol residue, or R9 and R10 may be combined together to form a carboxylic anhydride residue represented by xe2x80x94C(xe2x95x90O)OC(xe2x95x90O)xe2x80x94. Specific examples of alkyl represented by R9 or R10 include methyl, ethyl, propyl and the like. Specific examples of hydroxyalkyl represented by R9 or R10 include hydroxymethyl, 2-hydroxyethyl and the like. Examples of the alcohol residue represented by Z include alkyl with about 1 to 8 carbon atoms, which is unsubstituted or substituted, 2-oxooxolane-3- or -4-yl and the like. Possible substituents on the alkyl include a hydroxyl group, an alicyclic hydrocarbon residue and the like. Specific examples of carboxylic ester group represented by xe2x80x94COOZ include
methoxycarbonyl, ethoxycarbonyl, 2-hydroxyethoxycarbonyl,
tert-butoxycarbonyl, 2-oxooxolane-3-yloxycarbonyl,
2-oxooxolane-4-yloxycarbonyl,
1,1,2-trimethylpropoxycarbonyl,
1-cyclohexyl-1-methylethoxycarbonyl,
1-(4-methylcyclohexyl)-1-methylethoxycarbonyl,
1-(1-adamantyl)-1-methylethoxycarbonyl and the like.
Examples of monomers for leading the polymerization unit of formula (VII) include
2-norbornene, 2-hydroxy-5-norbornene,
5-norbornene-2-carboxylic acid, methyl
5-norbornene-2-carboxylate, t-butyl
5-norbornene-2-carboxylate, 1-cyclohexyl-1-methylethyl
5-norbornene-2-carboxylate,
1-(4-methylcyclohexyl)-1-methylethyl
5-norbornene-2-carboxylate,
1-(4-hydroxylcyclohexyl)-1-methylethyl
5-norbornene-2-carboxylate,
1-methyl-1-(4-oxocyclohexyl)ethyl
5-norbornene-2-carboxylate,
1-(1-adamantyl)-1-methylethyl 5-norbornene-2-carboxylate,
1-methylcyclohexyl 5-norbornene-2-carboxylate,
2-methyl-2-adamantyl 5-norbornene-2-carboxylate,
2-ethyl-2-adamantyl 5-norbornene-2-carboxylate,
2-hydroxyl-1-ethyl 5-norbornene-2-carboxylate,
5-norbornene-2-methanol
5-norbornene-2,3-dicarboxylic acid anhydride, and the like.
Depending on the kind of radiation for patterning exposure and the kind of group unstable to an acid, it is generally preferred that the resin used in the invention contains the polymerization unit having a group unstable to an acid in a range of 10 to 80% by mole based on the total resin. Particularly, when the polymerization unit of 2-alkyl-2-adamantyl (meth)acrylate represented by the formula (III) is used as the group unstable to an acid, it is preferred that the unit exists in 15% by mole or more based on the total resin. When, in addition to the polymerization unit having a group unstable to an acid, another polymerization unit hardly cleavable by the action of an acid, such as the polymerization unit of 3-hydroxy-1-adamantyl (meth)acrylate represented by the formula (IV), the polymerization unit of xcex1-(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (V), the polymerization unit of xcex2-(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (VI), the polymerization unit of 2-norbornene represented by the formula (VII), the polymerization unit of maleic anhydride represented by the formula (VIII) and the polymerization unit of itaconic anhydride represented by the formula (IX) as the polymerization units of the aliphatic unsaturated dicarboxylic anhydrides, are present, it is preferred that these units exist within a range of 20 to 90% by mole in total based on the total resin.
When a copolymer containing the polymerization unit of 3-hydroxy-1-adamantyl (meth)acrylate represented by the formula (IV) and/or the polymerization unit of xcex1-(meth) acryloyloxy-xcex3-butyrolactone represented by the formula (V) as well as the polymerization unit of 2-norbornene represented by the formula (VII) and the polymerization unit of the aliphatic unsaturated dicarboxylic anhydrides represented by the formula (VIII) or (IX) together with the polymerization unit having a group unstable to an acid containing a unit of 2-alkyl-2-adamantyl (meth)acrylate represented by the formula (III) is desired, it is usual to copolymerize a monomer mixture containing 10 to 80% by mole of a monomer having a group unstable to an acid, particularly 15% by mole or more of 2-alkyl-2-adamantyl (meth)acrylate for leading a unit of the formula (III), and 20 to 90% by mole in total of 3-hydroxy-1-adamantyl (meth)acrylate for leading the unit of the formula (IV) and/or xcex1-(meth)acryloyloxy-xcex3-butyrolactone for leading the unit of the formula (V) as well as a 2-norbornene compound for leading the unit of the formula (VII) and a monomer for leading the polymerization unit of aliphatic unsaturated dicarboxylic anhydride for leading the unit of the formula (VIII) or (IX). When a 2-norbornene compound and an aliphatic unsaturated dicarboxylic anhydride are used as monomers for copolymerization, it is preferred to use them in excess in view of the fact that these have a tendency of hardly polymerizing. Likewise, when a copolymer containing a polymerization unit of xcex2-(meth)acryloyloxy-xcex3-butyrolactone represented by the formula (VI) together with a polymerization unit having a group unstable to an acid is desired, it is advantageous to polymerize a monomer mixture containing 10 to 80% by mole of a monomer having a group unstable to an acid and 20 to 90% by mole of xcex2-(meth)acryloyloxy-xcex3-butyrolactone for leading the unit of the formula (VI).
It is also known that, generally in a chemical amplifying type positive resist composition, performance deterioration due to the deactivation of an acid associated with leaving after exposure can be reduced by adding a basic compound, especially a basic nitrogen-containing organic compound such as amines as a quencher. It is also preferable in the present invention that such basic compounds are added. Concrete examples of the basic compounds to be used as quenchers include the ones represented by the following formulae: 
wherein R11, R12 and R17 represent, independently each other, hydrogen, cycloalkyl, aryl or alkyl which may be optionally substituted with a hydroxyl, amino which may be optionally substitiuted with alkyl having 1 to 6 carbon atoms, or alkoxy having 1 to 6 carbon atoms; R13, R14 and R15, which are same or different from each other, represent hydrogen, cycloalkyl, aryl, alkoxy or alkyl which may be optionally substituted with a hydroxyl, amino which may be optionally substitiuted with alkyl having 1 to 6 carbon atoms, or alkoxy having 1 to 6 carbon atoms; R16 represents cycloalkyl or alkyl which may be optionally substituted with a hydroxyl, amino which may be optionally substitiuted with alkyl having 1 to 6 carbon atoms, or alkoxy having 1 to 6 carbon atoms; A represents alkylene, carbonyl, imino, sulfide or disulfide. The alkyl represented by R11 to R17 and alkoxy represented by R13 to R15 may have about 1 to 6 carbon atoms. The cycloalkyl represented by R11 to R17 may have about 5 to 10 carbon atoms and the aryl represented by R11 to R15 and R17 may have about 6 to 10 carbon atoms. The alkylene represented by A may have about 1 to 6 carbon atoms and may be straight-chained or branched.
Among the basic compounds as described above, 2,6-dialkylpyridine compound represented by the formula (X) is preferable for improving the storage stability of the resist: 
wherein R21 and R22 independently represent an alkyl having 1-4 carbon atoms. Concrete examples of the 2,6-dialkylpyridine compound include 2,6-lutidine, 2-ethyl-6-methylpyridine, 2,6-di-tert-butylpyridine, and the like. The 2,6-dialkylpyridine compound can be used alone or together with other basic compounds as a quencher.
The resist composition of the present invention preferably contains the resin in an amount in the range of 80 to 99.9% by weight, and the acid generator in an of 0.1 to 20% by weight based on the total amount of the resin and the acid generator.
In the resist composition of the present invention, the ratio by weight of the aliphatic sulfonium salt of the formula (I) to the onium salt selected from the group consisting of a triphenylsulfonium salt of the formula (IIa) and a diphenyliodonium salt of the formula (IIb) is preferably about 9:1 to 1:9, more preferably about 8:2 to 2:8.
When a basic compound is used as a quencher, it is preferably contained in an amount in the range of 0.0001 to 0.1% by weight based on the total solid component weight of the resist composition. The composition may also contain, if required, various additives such as sensitizers, dissolution inhibitors, resins other than resin, surfactants, stabilizers, and dyes so far as the objects of the present invention is not harmed.
The resist composition of the present invention generally becomes a resist solution in the state in which the above-described components are dissolved in a solvent to be applied on a substrate such as a silicon wafer. The solvent herein used may be one which dissolves each component, has an appropriate drying rate, and provides a uniform and smooth coating after evaporation of the solvent, and can be one which is generally used in this field. Examples thereof include glycol ether esters such as ethylcellosolve acetate, methylcellosolve acetate, and propylene glycol monomethyl ether acetate; esters such as ethyl lactate, butyl acetate, amyl acetate, and ethyl pyruvate; ketones such as acetone, methyl isobutyl ketone, 2-heptanone, and cyclohexanone; and cyclic esters such as xcex3-butyrolactone. These solvents can be used alone or in combination of two or more thereof.
The resist film applied on a substrate, and dried is subjected to an exposure treatment for patterning. Then, after a heat-treatment for promoting a protecting deblocking reaction, development by an alkali developer is conducted. The alkali developer herein used can be various kinds of alkaline aqueous solutions used in this field. In general, an aqueous solution of tetramethylammoniumhydroxide or (2-hydroxyethyl)trimethylammoniumhydroxide (so-called colline) is often used.